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FreeBSD 11.0 - man page for timer_create (freebsd section 2)

TIMER_CREATE(2) 					      BSD System Calls Manual						   TIMER_CREATE(2)

timer_create -- create a per-process timer (REALTIME)
POSIX Real-time Library (librt, -lrt)
#include <time.h> #include <signal.h> int timer_create(clockid_t clockid, struct sigevent *restrict evp, timer_t *restrict timerid);
The timer_create() system call creates a per-process timer using the specified clock, clock_id, as the timing base. The timer_create() sys- tem call returns, in the location referenced by timerid, a timer ID of type timer_t used to identify the timer in timer requests. This timer ID is unique within the calling process until the timer is deleted. The particular clock, clock_id, is defined in <time.h>. The timer whose ID is returned is in a disarmed state upon return from timer_create(). The evp argument, if non-NULL, points to a sigevent structure. This structure, allocated by the application, defines the asynchronous noti- fication to occur when the timer expires. If the evp argument is NULL, the effect is as if the evp argument pointed to a sigevent structure with the sigev_notify member having the value SIGEV_SIGNAL, the sigev_signo having a default signal number, and the sigev_value member having the value of the timer ID. The implementations supports a clock_id of CLOCK_REALTIME or CLOCK_MONOTONIC. If evp->sigev_notify is SIGEV_THREAD and sev->sigev_notify_attributes is not NULL, if the attribute pointed to by sev->sigev_notify_attributes has a thread stack address specified by a call to pthread_attr_setstack() or pthread_attr_setstackaddr(), the results are unspecified if the signal is generated more than once.
If the call succeeds, timer_create() returns zero and updates the location referenced by timerid to a timer_t, which can be passed to the per-process timer calls. If an error occurs, the system call returns a value of -1 and the global variable errno is set to indicate the error. The value of timerid is undefined if an error occurs.
The timer_create() system call will fail if: [EAGAIN] The calling process has already created all of the timers it is allowed by this implementation. [EINVAL] The specified clock ID is not supported. [EFAULT] Any arguments point outside the allocated address space or there is a memory protection fault.
clock_getres(2), timer_delete(2), timer_getoverrun(2), siginfo(3)
The timer_create() system call conforms to IEEE Std 1003.1-2004 (``POSIX.1'').
Support for POSIX per-process timer first appeared in FreeBSD 7.0.
January 12, 2009 BSD